Brewer's Yeast
Introduction and History For thousands of years the ability of yeast (Saccharomyces cerevisiae) to convert carbohydrates into carbon dioxide and alcohol has been exploited in baking and the brewing of alcoholic beverages (1). The earliest recorded uses of yeast for brewing dates back 4,000 years, as drawings found in ancient Egyptian ruins (2). French scientist Louis Pasteur proved than in fact the alcoholic fermentation process was conducted via living yeast cells and not through a chemical catalyst reaction (2) (3). During the late 18th century, two different yeast strains that are used in brewing had been identified: Saccharomyces cerevisiae ''(top-fermenting yeast) and ''S. carlsbergenesis ''(bottom-fermenting yeast), now referred to as ''S. pastorianus (1). S. cerevisiae ''is typically considered an 'Ale yeast', which can perform fermentation (aerobic respiration) at room temperature (+/- 10°F). Many different types of 'Ale yeast' strains are used today and are popular among homebrewers for their ease of use and hardiness. Bottom-fermenting yeast (''S. pastorianus)''' are typically considered to be 'lagering yeast', or yeast that are generally slower in activity although, perform fermentation at low temperatures (lagering-process). Each of these two classical pathways (ale's vs lager's) yield two distinct flavor compositions in beer, as well as effecting color, clarity, mouth feel and aromas''. ' Genetic Modifications and Brewing To date, 20 years has passed since the first research on the genetic modification of brewer's yeast was conducted, and still continues to this day. Although, no brewers currently use GM yeast strains, they rely on the natural hybridization of differing yeast strains to accomplish modifications (4). This process has been termed 'strain improvement'. Upon fermentation, brewer's yeast can yield a host of undesirable by-products such as diacetyls, sulfites and hydrogen sulfides that give beer a myriad of poor flavor profiles. Strain improvement is an attempt to remove or limit these off flavors sometimes incorporated into the beer via the aerobic respiration of the brewer's yeast. There has also been attempts to utilize other yeast strains that can properly metabolize various forms of sugars and carbohydrates more efficiently. The first classical methodology applied in the improvement of any genetic line would be to consider sexual mating, which allows for natural incorporation and combination of chromosomes. Lager brewer's yeast strains are often considered to have allopolyploid and also are homothallic. These two hurdles make it difficult to successfully breed and attain stable haploids within the yeast genome (5). Another classical genetic approach for improvement is the methodology termed protoplast fusion, in which hybridization occurs without consideration of mating types (6) . Using this technique, researchers were able to optimize the flocculation behavior of yeast via the hybridization of a highly-flocculent ''S. cerevisiae ''strain and a non-flocculent brewer's strain which ultimately lead to a highly-flocculent brewer's strain with good brewing characteristics (6). Methodologies in classical genetic modifications have yielded many variations in brewer's yeast, although with very low probability of correctly hybridizing desirable traits (7). Improvement of the beer making efficiency is of high importance to brewer's yeast manufacturers' and molecular techniques such as transformation and transfections have aided in this modification. Maltose is the main fermentable sugar with in a beer 'wort' (unfermented sugar rich mixture prior to the addition of yeast) and the yeast's ability to metabolize or more specifically transport it, is the rate limiting step (6). Extraneous transporter genes adjacent to the maltose (MAL) loci have been exploited in attempt to raise extracellular levels of these transporters to in turn enhance the fermentation efficiency of the brewer's yeast (8). While this is simply one example of using the easily manipulated yeast genome to enhance the efficiency of beer production, there are many more examples in literature of genomic alterations of brewer's yeast to enhance desired phenotypes. Some of the aforementioned include but are not limited to (6) : *Improvement of carbon utilization *Gained removal of yeast biomass from beer *Increased flocculation *Yeast more resistant to fluxes in pH, ethanol concentrations and oxygen levels *Enhanced filterability *Reduction of 'off-flavors' ie. Diacetyl, hydrogen sulfides, esters, etc. *Supplementation of extraneous enzymes such as beta and alpha amylase effecting the transcription of genes within the yeast genome Brewer's yeast over the years has been successfully optimized via hybridization events to attain desirable beer brewing features and limit the non-desirable phenotypes. There are certainly caveats within this procedure as brewing is a process that is generally not uniform in temperature, pH, oxygen levels, sugar levels, etc across batches. Many commercial brewers have spent millions of dollars in attempt to replicate their procedures to the best of their ability, although, even minor changes noted above can have profound effects upon the fermentation output of yeast. With GMOs gaining significant notoriety in the government and mainstream media, it is unlikely that other molecular techniques will be further used in attempt to enhance brewer's yeast strains. Conversely, as microbreweries and homebrewing alike gain popularity, yeast manufacturers such as Wyeast labs continue to create unique strains of yeast that produce distinct flavors and profiles. References #http://en.wikipedia.org/wiki/Yeast #http://science1.nasa.gov/science-news/science-at-nasa/msad16mar99_1b/ #Barnett JA (2003) Beginnings of microbiology and biochemistry: the contribution of yeast research. Microbiology 149(3):557-567 #http://www.ncbe.reading.ac.uk/ncbe/gmfood/yeasts.html #Saerens SM, Duong CT, & Nevoigt E (2010) Genetic improvement of brewer's yeast: current state, perspectives and limits. Appl Microbiol Biotechnol 86(5):1195-1212 #Hansen J & Kielland-Brandt M (2003) Brewer’s yeast: genetic structure and targets for improvement, in Functional Genetics of Industrial Yeasts, Topics in Current Genetics, ed Winde J (Springer Berlin Heidelberg), Vol 2, pp 143-170 #Attfield P & Bell PL (2003) Genetics and classical genetic manipulations of industrial yeasts, in Functional Genetics of Industrial Yeasts, Topics in Current Genetics, ed Winde J (Springer Berlin Heidelberg), Vol 2, pp 17-55 #Vidgren V, Ruohonen L, & Londesborough J (2005) Characterization and functional analysis of the MAL and MPH Loci for maltose utilization in some ale and lager yeast strains. Appl Environ Microbiol 71(12):7846-7857